2 colors of grout would not be appealing to me and in fact 2 different sizes of subway tile IMO would look like you ran out of one size. Keep it simple choose one size tile and use it everywhere I like large format tile in showers with a grout that matches the tile to keep it all simple and easy to clean In fact I like 12 x 24 tile on the bathroom floor and the same tile on the shower walls, with a smaller tile for the shower floor

I would use the 8 x 16. When the width is half the length, you won't have to offset it by a third, and I think that's a better look for a traditional bath. One of the tile setters here said to buy tile with that ratio when you choose large format subway because the tile won't have any bowing. I really hate the look of lippage on wall tile.

I would use a smaller tile for the top section in the same ratio--3x6 or 4x8. I found it helpful to draw out my tile ideas on graph paper and to mock it up with paper cut to tile size.

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Just NO.

Mismatched tile sizes? Mismatched grout? Why would you do this to yourself?

The OP never said he was planning on using mismatched grout.